1. Field of Invention
This invention relates to a particle component analyzing apparatus which uses a microwave induced plasma to perform element analysis of, for example, particles which float in a clean room, and to a particle composition determining method for obtaining accurate equivalent particle diameters from information obtained using such apparatus and for determining the composition of the particles.
2. Description of the Prior Art
FIG. 1 shows a conventional particle component analyzing apparatus using a microwave induced plasma. The apparatus comprises a disperser 1 having therein a filter 2, with solid particles to be measured (not shown in the drawing) adhereto thereto, and an aspirator 3 which draws up solid particles adhered to the filter 2, and feeds the particles through valve 7c into one end of a discharge tube 4. Air is removed from inside disperser 1 by a suction pump 5 and the He gas is introduced through an inlet 8 and valve 7b to maintain a pressure slightly higher than atmospheric pressure. A carrier gas (e.g. He) is applied through inlet 9 and valve 7a.
A microwave source 13 introduces microwaves into a cavity 17. A detection window 16 is disposed at the other end of discharge tube 4. An optical window 17 is disposed facing detection window 16. A focusing unit 18 is provided comprising a concave mirror 18a and a reflector 18b. The emission spectrum caused by the microwave impinging on the particles adhered to the filter causes the elements of the particles to emit an emission spectrum having a plurality of wavelengths. The emission spectrum is guided through slit 19 after being reflected by reflector 18b and then is guided into a signal processing section 20. Signal processing section 20 comprises four monochrometers 20b, each of which receives emission spectrum through one of four optical fibers 20c. The outputs of the monochrometers 20b are applied to central processing unit (CPU) 20a.
In the FIG. 1 apparatus, microwaves having a frequency of 2.45 GHz are generated by source 13 and are applied to cavity 14, and a plasma of approximately 4000.degree.K. is created in discharge tube 4.
Solid particles guided into discharge tube 4 from dispenser 1 are atomized, ionized and excited in the plasma, and emitted as an emission spectrum as they are reduced to their base states. This emission spectrum is led out of discharge tube 4 in the axial direction thereof, then guided through optical window 17 into focusing unit 18, wherein the emission spectrum is focused, and then passed through slit 19, then separated into different wavelengths by monochrometers 20b, and then signal processed by CPU 20a. In this manner, the elements contained in the specimen particles are measured and displayed. Monochrometers 20b are provided with optoelectrical converters 20d, which output electrical signals corresponding to the strength of light of the selected wavelengths. Amplifiers 21, which are ordinary amplifiers , when FIG. 1 is considered to show a prior art apparatus, amplify the output signals from optoelectric converters 20d. The amplifiers 21 are disposed after the converters 20d. The sizes of the particles are classified according to the strength of the output signals from the amplifiers 21, such as, for example, three classes of large, medium and small.
It should be mentioned hereat that FIG. 1 shows amplifier 21 as being an ordinary amplifier when the apparatus is a conventional apparatus. When FIG. 1 is illustrative of the invention, the amplifier 21 is a cube root amplifier. The drawing distinctly shows that for the conventional apparatus, an ordinary amplifier is used as amplifier 21, and that for the illustrative embodiment of the invention, a cube root amplifier is used as amplifier 21, and FIG. 1 is to be understood be signify such dual meaning.
Returning to FIG. 1, filter 2 has a predetermined surface area, and aspirator 3 scans the filter a plurality of times, for example, 15 times, as shown in FIG. 2, and draws up the same quantity of particles each time. It is assumed that the particles, including a plurality of elements, are distributed throughout the filter surface, with the elements being present in the same proportions in each particles, according to the amounts present in the selected sample. That is to say, the elements drawn in by the multiple scans are assumed to be drawn up in the same proportion with each scan, and each monochometer is set to one wavelength to analyze one element during each scan. Elements of which the emission spectrum wavelenghts have been determined include, for example, Al, Fe, C, P, Si, Cu, B, K, Na, Ni, Cr, Ca, C1, F, N, W, Ti, Mo, Mg, Zn, Au, Co, Mn, Pb, 0, S, and Br.
Quality deterioration in semiconductor manufacturing, for example, is caused by inaccurate measurement of the elements, that is composition of an ingredient, and by inaccurate measurement of size or weight. Thus, by simply analyzing the average quantity of an element would not be sufficient to insure against quality deterioration.